Multiple zone air conditioning system

ABSTRACT

A multiple zone air conditioning system is disclosed having a control system in which heating or cooling equipment is controlled according to the maximum and minimum zone air conditioning loads. The control system is constructed using electronic components which are protected against damage due to improper installation.

United States Patent 91 Demaray Jan. 29, 1974 [54] MULTIPLE ZONE AIR CONDITIONING 3,394,754 7/1968 Schneeberg et a1. 165/27 X SYSTEM 3,568,760 3/1971 Hogel 165/22 3,504,848 4/1970 Foster et a1 165/22 X Inventor: Merlin ay, C l s, 3,645,325 9/1970 Lorenz 165/22 x Ohio [73] Assignee: Ranco Incorporated, Columbus, Primary Exami"er"Manuel Amonakas Ohio Attorney, Agent, or FirmWatts, Hoffman, Fisher & Heinke, Co., L.P.A. [22] Filed: Nov. 30, 1971 [21] Appl. N0.: 203,329 [57] ABSTRACT A multiple zone air conditioning system is disclosed 52 US. Cl 165/2, 165/22, 165/27 having a control y em in hich heating or cooling [51] Int. Cl F24f 3/08 equipment is controlled cor ing to the maximum [58] Field of Sear h 165/22, 1 2, 50, 30, 27 and minimum zone air conditioning loads. The control system is constructed using electronic components [56] References Cited which are protected against damage due to improper UNITED STATES PATENTS mstallatlm 3,567,115 12/1970 Nelson 165/22 X 7 Claims, 4 Drawing Figures ZONE ZONE 3 205g 20/ /55 zogg; 20g; 201g; 20%;

PATENTEBannzs m4 3; 788 386 sum 2 or 3 v m mz m J W m MM m an m0 a C m t m m m 1 3 4 1 l I I .I II I I u N 3% \SRSm WERE M Q Q 3 8: Qt ME A Q 3 3 u MULTIPLE ZONE AIR CONDITIONING SYSTEM CROSS REFERENCED COMMON ASSIGNED APPLICATIONS US. Pat. application Ser. No. 87,963, filed Nov. 9, 1970 entitled METHOD & SYSTEM FOR CON- TROLLING AIR TEMPERATURE IN AN AIR CON- DITIONING ZONE, by Russell G. Attridge, Jr. now U.S. Pat. No. 3,745,778.

US. Pat. application Ser. No. 168,596 entitled HY- DRAULIC ACTUATOR by Robert E. Brooks and Robert T. Marks now US. Pat. No. 3,707,844.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to air conditioning systems and more particularly relates to air conditioning systems for supplying heated or cooled air to a plurality of air conditioned zones.

2. The Prior Art Air conditioning systems for heating and/or cooling multiple zone buildings have been proposed in which the air heating and cooling equipment is part of a selfcontained unit. One or more of these units are situated within or adjacent the building and air is delivered to the zones through ducts. In order to assure compatibility of the air conditioning units and control systems which govern their operation it has been proposed to provide factory installed control systems in the units and to employ compatible thermostatic sensors in the zones and/or in ducts associated with the zones.

Multiple zone air conditioning systems have employed controls which govern operation of air heating or air cooling apparatus in accordance with the maximum sensible air conditioning load present in any one of the zones. The magnitude of the sensible air conditioning load in any zone is a function of the difference between a set point temperature for the zone and the sensed air temperature in the zone. In such systems the maximum air conditioning load is indicated by some sort of signal, mechanical or electrical, which has a maximum or a minimum value when compared to the signals from all other zones.

Some prior art systems of the type referred to employed electrically operated apparatus for governing operation of air heating or air cooling equipment in response to output signals from electrical temperature responsive circuits. The circuits were constructed so that when all zones were at or close to the system set point temperature a null or no signal condition existed. If zone temperatures increased above the set point level, a signal was produced in proportion to the highest sensed zone air temperature. If zone temperatures decreased below the set point temperature a signal of opposite polarity was produced in proportion to the deviation between the temperature sensed in the coolest zone and the set point temperature. Thus, when the air temperatures in all zones were either above or below the set point temperature, the air conditioning equipment responded only to the demand of the most heavily loaded zone. Load signals from the remaining zones had no effect on the equipment and were thus, in effect, disregarded.

Controlling the air conditioning systems with such controls usually entailed cycling the heating or cooling equipment and produced corresponding fluctuations in the zone temperatures. In installations employing electric resistance heating units for heating air ducted into the zones, heater cycling tended to maximize the energy consumption of the heaters in satisfying the demand from the coldest zone. This tended toward relatively high system operation costs due to the electrical energy consumption attendant their usage. Moreover, the heat produced was sometimes excessive for zones having relatively small heating loads.

The same situation obtained when all zones were at temperatures greater than the set point level, and refrigeration units were operated to cool the zones.

Control systems of the types referred to have sometimes been constructed so that they were difficult to install properly and malfunctions in system components were not readily detected, diagnosed and serviced. Generally, the zone air temperature sensor units and the air conditioning units are installed at different stages of the building construction. This requires completion of the wiring between the air conditioning units and the temperature sensor units when all components of the air conditioning system have been installed. In control systems employing electronic components, it has been possible to improperly wire the zone sensor units and the air conditioning units together which has caused malfunctions in the control systems. In some situations, improper wiring leads to destruction of some of the electronic components. Furthermore, malfunctions have been difficult to discover and diagnose which has lead to frequent service calls and unsatisfactory performance of the air conditioning equipment.

SUMMARY OF THE INVENTION The present invention provides a new and improved control system for a multiple zone central air conditioning system which is effective to enable operation of the air conditioning system according to the demands for conditioned air from the zones having both the greatest and the smallest air conditioning loads. The new system also enables zone sensor units to be wired into the system with minimal risk of damage to electronic components as a result of improper wiring. The new system further enables the existence and locations of certain types of malfunctions and improper wiring connections between zone sensor units and associated control system components to be quickly discovered. In its preferred form, the new system is employed in a multiple zone building which is provided with an air conditioning system for heating and/or cooling air directed to zones in the building. The air conditioning system includes a blower, air heating equipment and air cooling equipment. The air cooling equipment is preferably constructed to provide multiple stages of air cooling heat exchangers. The air heating equipment is likewise constructed to provide multiple stages of air heating heat exchangers.

The new control system comprises controller units for the air heating and cooling equipment and zone sensor units governing operation of the controller units. Each stage of the air heating and air cooling equipment is operated by a respective voltage responsive controller unit. The controller units are operated by analog condition dependent command signals produced by the zone sensor units associated with respective ones of the zones.

The magnitude of the command signal produced by a given zone sensor unit is, in general, proportional to the air conditioning load on the zone. When a zone is at or close to a zone set point temperature, the command signal produced by the respective sensor unit is ineffective to operate any heating or cooling equipment controllers. When the zone temperature varies a predetermined amount from the set point level the command signal level changes and causes operation of a heating controller or a cooling controller depending on the sense direction of the zone temperature change relative to the set point temperature.

The individual controller units are responsive to different command signal voltage levels so that the cooling or heating equipment is operated in stages as a command signal voltage varies proceeding away from the set point voltage signal level in a positive or a negative sense respectively. 7

An important feature of the invention resides in the construction and arrangement of the system to enable the command signals from each zone to be continuously compared and wherein both the maximum command signal and the minimum command signal are continuously applied to appropriate controller units. The command signals are direct current signals which are of the same polarity with respect to a reference voltage (e.g., positive with respect to ground) and which can vary in a positive or negative sense relative to desired zone set point voltage levels. Hence, there is no null or zero command voltage level which corresponds to a set point temperature level.

By continuously utilizing both the maximum and minimum zero sensor command signals, it is possible to provide cooling for zones having air temperatures above a set point level while simultaneously heating zones having air temperatures below the set point level. Furthermore, when all the zones are above or below the set point level, control of the cooling or heating equipment, respectively, is accomplished using both the minimum and maximum command signals.

Controlling the air conditioning equipment according to both the highest and lowest zone signals, when all zones are above or below set point temperatures, reduces cycling of the heating or cooling equipment and tends to reduce temperature fluctuations in all the zones. As an example, the new control system enables operation of a stage of the heating equipment in response to the command signal from the most lightly loaded zone when all zones are close to but below the set point temperature level. This stage of the heating equipment is preferably a relatively small capacity secondary or auxiliary type heater which provides a relatively low level of heat input to all the zones thus tending to balance the heating loads in all zones. If the air temperature in one or more zones decreases a predetermined amount below the set point temperature, one or more additional stages of the primary heating equipment will respond to the command signal from the coldest zone so that air supplied to all zones is heated according to the requirements of the coldest zone.

The secondary heater stage of the heating equipment continues to be responsive to the load in the warmest zone. Hence, where the secondary heater stage controlled by the warmest zone provides a low cost source of heat, e.g., an oil or gas fired furnace, a source of waste heat from the building, or a relatively low capacity electric resistance heater, the air directed to all zones is heated in part by the low cost heat source operated by the command signal from the warmest zone. This reduces cycling of high operating cost heating equipment, such as larger electric resistance heaters, while retaining the advantages of such systems, i.e., quick response, ease of installation and cleanliness.

The operation of the control system when cooling is required by all zones is analogous to operation on the heating cycle. The low cost cooling source could be provided by a relatively small capacity mechanical refrigeration unit or cooling tower water, for example.

Another important feature of the invention resides in the construction of a multizone temperature control system which, if improperly installed, protects components of the system from damage due to the improper installation as well as enabling relatively rapid diagnosis and correction of the installation errors. The new system employs electronic components which are connected to a regulated DC. power supply. The circuitry providing the command signals includes a diode connected between its power input terminals. When the command signal circuitry is properly wired to the power supply, the diode is biased to a nonconducting condition by the power supply. However, when the signal circuitry power input terminals are improperly connected to the power supply, the diode conducts and shunts the signal circuitry. This prevents electronic components in the signal circuitry from being damaged due to the reverse polarity connections since the voltage across such components is limited to the forward voltage drop across the diode.

Another feature resides in the power supply circuitry which is constructed so that when the current load across the output is substantially increased, e. g., when a diode in the command signal circuitry conducts due to improper wiring, the power supply output is substantially reduced. Hence, if any of the command signal circuits are improperly connected to the power supply the power supply output is ineffective to operate the system and this fact is readily apparent to the equipment installer. The proper wiring connections can then be made without damage to the system components.

During operation of the system, if any components of the signal circuits or controller circuits malfunction or fail in such a way that the current load on the power supply is substantially increased, for example due to a short circuit, the power supply output voltage is reduced and the system ceases to operate. To service the system, individual circuits are successively disconnected from the power supply. When the circuit containing the malfunctioning component is disconnected, the proper power supply voltage and current are reestablished and the control system is reactivated. The malfunctioning circuit is thus identified and can be repaired or replaced.

Other features and advantages of the invention will become apparent from the following detailed description made with reference to the accompanying drawings which form a part of the specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an air conditioning system embodying the present invention;

FIG. 2 is a schematic view of a part of the air conditioning system shown in FIG. 1;

FIG. 3 is a schematic diagram of part of an electrical control system for the air conditioning system; and,

FIG. 4 is a graphic illustration depicting operation of the system.

DESCRIPTION OF A PREFERRED EMBODIMENT An air conditioning system for a multiple zone building is illustrated in FIG. 1 of the drawings. The system 10 comprises air Conditioning equipment, generally designated at 12, for supplying conditioned air to zones l3 in the building under the governance of a control system 14.

The air conditioning equipment 12 is effective to heat and/or cool air directed to the building and may take the form of a rooftop type heating and cooling unit. The equipment 12 is contained in the housing and comprises a blower 22, air chilling heat exchanger units 24, 26 and air heating heat exchanger units 28, 30. The housing 20 defines an outside air inlet 32 and a return air inlet 34 through which air is drawn into the housing by the blower 22. Air moving through the housing passes across the heat exchangers so that temperature conditioned air is directed from the housing 20 through chilled air outlets 36, 38, 40 which communicate with zones l-3 respectively and heated air outlets 42, 44, 46 which also communicate with the zones 1-3, respectively. Zone air damper units 48-50 are located in the heated and chilled outlets and are actuable to control the quantities of heated and/or chilled air flowing to any particular zone. A damper unit 52 is also associated with the outdoor air inlet and the return air inlet for governing the proportions of return and ventilation air circulating through the zones.

THE HEATING AND COOLING EQUIPMENT In the illustrated embodiment, multiple stages of heating and cooling are provided. Referring now to FIG. 2, the air cooling heat exchanger unit 24 comprises three separate air cooling heat exchangers 24a, 24b, 240 which are individually operable. The heat exchangers 24a-c are preferably defined by evaporators of conventional compressor-condenser-evaporator refrigeration units which have not been illustrated. The air cooling heat exchanger unit 26 is, for convenience, referred to as a secondary heat exchanger and is illustrated as a single stage unit providing a low cost source of cooling. The unit 26 is preferably a relatively small compressor-condenser-evaporator refrigeration unit, schematically shown, but may be a suitable fin-tube heat exchanger in which cooling tower water circulates.

The heat exchanger unit 28 includes four separate air heating heat exchangers 28a-d each of which is individually operable so that the amount of heat transferred into air passing through the unit 28 can be varied stepwise according to the number of heater stages which are energized. The heating stages are each preferably electrical resistance heaters however other suitable sources of heat can be provided. The heat exchanger unit is, for convenience, referred to as a secondary heater and provides relatively low cost source of heat. For example, the heat could be provided by process steam or the unit could be formed by a heat exchanger of a conventional oil or gas fired furnace in the building. This unit is illustrated as a relatively small electric resistance heater.

THE CONTROL SYSTEM The control system 14 is constructed and arranged so that most of the components of the control system may be constructed integrally with the housing 20 and hence can be supplied by the equipment manufacturer as factory installed components. The remaining components of the control system are associated with the individual zones.

In the illustrated embodiment, the control system 14 includes zone temperature sensing units 60, 62, 64 disposed in the zones 1-3, respectively, and a control assembly 66 located with the air conditioning equipment 12 remote from the zones. The zone sensing units are wired to the components of the assembly 66 when the equipment 12 is installed in the building.

The control assembly 66 comprises a regulated DC. power supply 78 for the control system 14, controllers for the various heat exchanger units, and logic circuitry for applying command signals from the zones to the controller units. Referring now to FIG. 2, the control assembly 66 includes voltage responsive heater controllers 80-83 for the heater stages 28a-d, respectively, voltage responsive refrigeration unit controllers -87 for the cooling stages 24a-c, a voltage responsive secondary heater controller for the secondary heater 30 and a voltage responsive secondary cooling controller 92 for the secondary cooler 26. Command signals from the zone sensing units are distributed to appropriate controllers through logic circuitry generally designated at 94. The housing 66 also includes voltage responsive damper actuator controllers 96-98 which control the zone damper units 48-50, respectively, and a voltage responsive damper actuator controller circuit 100 for controlling the ventilation damper unit 52.

THE POWER SUPPLY The regulated DC. power supply 78 is illustrated in FIG. 3. The power supply 78 is preferably connected across a relatively low voltage A.C. power supply 103, e.g., 24 V.A.C., which is provided by the secondary winding of a conventional step-down transformer, not illustrated. A rectifier is formed by a diode and capacitor 106 which cooperate to provide rectified filtered direct current.

A regulator circuit 108 is connected across the rectifier to smoothen the output voltage from the rectifier and provides a closely regulated substantially steady state direct current power supply output across power supply output terminals 110, 112. The preferred regulator circuit comprises transistors 120, 122 which cooperate to smoothen the output from the rectifier. The base electrodes of the transistors 120, 122 are both connected to a reference voltage provided at a junction 123. The collector-emitter circuits of the transistors 120, 122 are connected in parallel between the rectifier and the output terminal 110. The transistor is an NPN transistor which provides for primary regulation of the output voltage from the rectifier while the PNP transistor 122 functions to diminish positive going ripple voltages which would otherwise be passed through the collector-emitter circuit of the transistor 120.

The reference voltage at the junction 123 is controlled by transistors 124, 126 and associated circuitry. The transistor 126 is connected to function as a Zener diode and provides a constant voltage at the emitter of the NPN transistor 124. The base of the transistor 124 is connected to a potentiometer 130 which in turn forms part of a resistance network across the output of the voltage regulator circuit. The junction 123 is connected between the collector of the transistor 124 and series connected resistors 131, 132. The voltage level at the junction 123 is therefore determined by the voltage drop across the resistors 131, 132 and the voltage drop across the transistors 124., 126. The potentiometer 130 is adjusted to control the conductivity of the transistors 124, 126 and hence its adjustment determines the voltage at the junction 123 and the output voltage of the power supply under normal operating conditions of the system.

If the current load on the power supply is substantially increased, for example, by a short circuit or other equivalent malfunction, the power supply output current and voltage is substantially diminished and the control system is disabled. This is accomplished by operation of an NPN load sensing transistor 133 which has its base and emitter electrodes connected across a resistor 134 which is connected between the output terminal 110 and the emitter of the transistor 120. When the current load is substantially increased, current through the resistor 134 is substantially increased which in turn causes the transistor 133 to turn on. The collector of the transistor 133 is connected between the resistors 131, 132 and to the emitter of the transistor 122. Accordingly, conduction of the transistor 133 shunts the resistor 132 and the transistors 124, 126 so that the voltage at the junction 123 is reduced. This reduces the voltage level at the base of the transistor 120 so that the transistor 120 tends to turn off. Conduction of the transistor 133 also starves the emitter-collector circuit of the transistor 122 so that both the transistors 120, 122 are cut off or permitted to conduct only a limited amount of current. The voltage drop across the resistor 131 and the transistor 133 is relatively large so that when the transistor 133 is conductive the output current from the power supply is substantially reduced.

The substantial reduction in the output voltage from the power supply due to the increased current load disables the control system so that the presence of a malfunction is readily apparent. Furthermore, when the malfunctioning component is disconnected from the power supply, the power supply output voltage immediately increases to its normal level thus enabling the source of the malfunction to be quickly isolated for servicing or replacement.

The controller units in the preferred embodiment operate relay contacts which in turn control solenoid actuators associated with the heating and cooling equipment. The solenoid actuators are connected to the AC. power supply 103 for the control circuitry and when switched on often have a high inrush current. This inrush current may cause a momentary collapse of the AC. supply voltage. To prevent this voltage collapse from adversely effecting the control circuitry, a capacitor 136 is connected across the power supply terminals 110, 1 12. The capacitor 136 stores current so that when the voltage collapse occurs the capacitor supplies adequate current to operate the control circuitry until the A.C. power supply voltage is restored. A diode 137 is connected between the regulating circuitry 108 and the capacitor 136 to prevent the capacitor 136 from discharging through the regulating circuitry 108.

THE ZONE SENSING UNITS The zone sensing units produce temperature responsive D.C. analog comrnand signals each of which is transmitted to the assembly 66. The zone sensing units 60, 62, 64 are each substantially identical and each includes a duct air temperature sensor 70, a zone air temperature sensor 72, a temperature set point adjustment element 74 and an amplifier 76 (see FIG. 1). The sensors 70, 72 and set point adjustment element 74 each produce analog temperature responsive voltage signals which are algebraically combined and applied to the input of the amplifier 76. The amplifier 76 is preferably a linear amplifier which produces an output or command voltage signal from the zone which is proportional to the input signal. The output signal from the amplifier 76 is transmitted to the control assembly 66.

The zone sensing units 60, 62, 64 are preferably all the same and are substantially similar to the circuitry which is disclosed in the cross referenced application to Attridge. The zone sensing units differ from those in the cross referenced application in that the output or command signal from the amplifier of each unit preferably varies between 2 and 22 volts positive relative to the reference voltage at the power supply terminal 1 12.

Each amplifier 76 includes an output stage 76a comprising emitter follower connected transistors 142, 144 which are employed to reduce the output impedance of the amplifier as Well as to provide increased command signal current from the zone sensor units. This construction is preferred since the control assembly 66 may be located remote from the zone sensing units and these may be connected by long leads. F urthermore, the low impedance signals transmitted to the control assembly from the zone sensing units resists antenna pickup of electrical noise which might otherwise occur with a high impedance signal.

The emitters of the transistors 142, 144 are connected to a zone sensor output terminal 146. The outputs 146 of the zone sensor units 60, 62, 64 are connected to the logic circuitry 94 in the control housing 66 via conductors 162-164, respectively, to provide separate inputs to the logic circuitry.

The zone sensing units 60, 62, 64 are preferably constructed using solid state integrated circuit elements which can be damaged if the connections between these units and the power supply output terminals 110, 112 are reversed. To prevent damage to the zone sensing units and enable prompt discovery of the improper wiring, diodes 165 are connected across the inputs of the zone sensing units. When a zone sensing unit is properly connected to the power supply terminals 110, 112, as is shown in FIG. 3, the diodes 165 are reverse biased and nonconducting. When the connections between any of the zone sensing units and the power supply terminals are inadvertently reversed, the diode 165 associated with that zone sensing unit becomes forwardly biased and conducts. Conduction of any of the diodes 165 shunts the associated zone sensing unit and obviates the possibility of damage to the zone sensing circuitry which might occur due to the reverse polarity connection to the power supply terminals.

Moreover conduction of a diode 165 creates, in effect, a short circuit across the power supply output terminals 110, 112. This reduction in current load on the power supply reduces the power supply output, as described above, and prevents operation of the control system. The existance of one or more improperly connected zone sensing units is thus readily apparent to the equipment installer. At this juncture the zone sensing units are disconnected from the power supply one at a time until the power supply output is restored to its op erating level indicating that the improperly wired zone sensingvunit has been disconnected. The undamaged zone sensing unit can then be properly reconnected to the power supply.

THE LOGIC CIRCUITRY The logic circuitry 94 has two output conductors 166, 168 and enables the maximum and minimum command signals from the Zone sensing units to be continuously transmitted to the output conductors 168, 166, respectively. The output conductor 166 is connected to the primary hcutcr controllers 80-83 and to the secondary cooler controller 92. The output conductor 168 is connected to the primary cooler controllers 85-87 and to the secondary heater controller 90.

The maximum command signal level is produced by the zone sensing unit in the zone having the highest temperatures. As shown in FIG. 3 the logic circuitry includes diodes 170, 172, 174 which have their respective anodes connected to the conductors 162-164. The cathodes of the diodes 170, 172, 174 are all connected to the output conductor 168 of the logic circuitry. The conductor 168 is connected to the power supply terminal 112 through a resistor 178 so that the minimum voltage level which could be produced at the output terminal 168 is approximately 2 volts above the level of the power supply terminal 112.

The voltage level at the output terminal 168 is always substantially the same as the maximum command signal voltage level produced by any one of the zone sensing units. For example, referring to FIG. 3, if the command signal voltage at the output terminal 146 of the sensing unit 60 for zone 1 is more positive than the command signals from the remaining zone sensing units, the command signal voltage from the zone 1 sensing unit is applied across the output terminal 146 of the zone sensing unit and the power supply terminal 112, through the conductor 162, the diode 170, the output conductor 168 of the logic circuit and the resistor 178. Since the command signal from the zone 1 sensing unit 60 is greater, i.e., more positive, than the command signals from the sensor units 62, 64, the diodes 172, 174 are biased to nonconductive conditions and the command signal from the zone sensing unit 60 is supplied to the controllers 85-87 and 90 without being affected by the command signals produced by the units 62, 64.

If one of the other zone sensing units produces a greater command signal than that of the sensor unit 60, the command signal from that zone sensing unit is provided at the output terminal 168 and the cooling controllers 85-87 and the heater controller 90 are controlled by that command signal.

The minimum command voltage signal, i.e., the least positive signal, is produced by the zone sensing unit in the coolest zone and this signal is continuously applied to the heater controllers 80-83 and the secondary cooling controller 92. The logic circuitry 94 includes diodes 180, 182, 184 which have their cathode electrodes connected to the respective conductors 162-164. The anode electrodes of each of the diodes 180, 182, 184 are connected together at the output conductor 166 of the logic circuitry. The output conductor 166 is connected to the heater controllers 80-83 and the secondary cooling controller 92 and is also connected to the power supply terminal 110 through a resistor 188. The diodes 180, 182, 184 enable the lowest magnitude command signal from the zone sensor units to be continuously applied to the controllers 80-83 and 92. For example, if the zone sensing unit 60 provides the lowest of minimum command signal, current flows from the output conductor 166 of the logic circuitry through the diode 180, the conductor 162, the output conductor 146 of the sensing unit 60, the transistor 144 and to the power supply terminal 112. The remaining diodes 182, 184 are reverse biased and nonconductive since the voltages at their cathode electrodes are greater than the voltage at their anode electrodes.

This low level signal is continuously applied to the heater controllers and secondary cooler controller. Should another of the zones become cooler than zone I, the output or command signal from the coolest zone sensing unit would become the low voltage level at the output conductor 166 and the diode 180 of the zone 1 sensing unit would be reverse biased.

THE CONTROLLER UNITS The refrigeration unit controllers -87 are all voltage responsive devices which are individually operated to operate the associated refrigeration units at preselected command signal levels. The controllers 85-87 are preferably identical and accordingly only the controller 85 is illustrated and described in detail.

The controller 85 comprises a differential amplifier including transistors 190, 192. The base of the transistor 192 is connected to a resistance network which is adjusted so that the transistor 192 is normally in a conductive state when the maximum command signal voltage is below a preselected level. Conduction of the transistor 192 maintains the transistor 190 in a nonconductive state. When the command signal voltages from the output terminal 168 of the logic circuitry 94 rises to the preselected level, the transistor 190 becomes conductive and cuts off conduction of the transistor 192. A feedback resistor 193 is connected between the collector of the transistor 192 and the base of the transistor 190 to increase the abruptness of the switching action of the transistors 190, 192.

The transistor 190 has its collector electrode connected to the power supply terminal through a relay coil 194 so that when the transistor conducts, the relay coil 194 is energized and the relay contacts controlling the associated refrigeration unit are closed to initiate operation of the refrigeration unit.

The base of the transistor 190 is connected to the output junction of a high impedance voltage divider comprised of resistors 195a, 195b. The high impedance input to the transistor 190 assure that the controller unit 85 draws minimal command signal current.

The controller unit 85 is bistable so that the associated refrigeration unit remains energized as command signal levels are reduced below the predetermined level. This function enables the zones to cool down sufficiently to avoid rapid cycling of the refrigeration units. For example, if the controller 85 is operated at a command voltage level of 17 volts, it may cease to operate 14 or 15 volts.

The controller units 86, 87 are constructed to energize their associated refrigeration units at progressively higher command voltage levels so that staging of the refrigeration units is accomplished in accordance with demand for zone cooling. The units 86, 87, like the unit 85, have high input impedance, are bistable and remain energized as command voltages fall below their actuating levels.

Another important feature of the controller units 85-87 and 90 resides in the fact that they are failsafe in operation. That is, if the zone sensing units are for some reason disconnected from the logic circuitry, the absence of any command signal results in the refrigeration units and secondary heater being deactivated. This function is assured due to the connection of the conductor 168 to the power supply terminal 112 through the resistor 178 which maintains the voltage level at the inputs of the controller units 85-87 and 90 at about the set point level so that none of the controllers 85-87 and 90 can be activated. it should be pointed out that the resistor 178 permits the command voltage level to be reduced well below the set point level when one or more zone sensing units are connected to the logic circuitry due to the conduction of the amplifier output stages 76a which tend to shunt the resistor 178.

The heater controllers are voltage responsive devices which are individually operated in response to preselected command voltage levels. The heater controllers 80-83 are all the same and accordingly only the controller 80 is illustrated and described. The controller 80 comprises a differential amplifier consisting of transistors 196, 198. The transistors 198 is normally conductive at command signal voltage levels greater than a preselected level and the transistor 196 is cut off. When the lowest command voltage level reaches the preselected low level, the transistor 196 becomes conductive and cuts off conduction of the transistor 198.. The transistor 196 has its collector electrode connected to the power supply terminal 112 through a relay coil 200. Thus, conduction of the transistor 196 energizes the relay coil 200 to close the contacts of the associated heater unit and energize the heater.

A feedback resistor 202 is connected between the collector of the transistor 198 and the base of the transistor 196 to facilitate abrupt switching action between these transistors. The base of the transistor 1.96 is connected to the outputjunction ofa high impedance voltage divider formed by resistors 204a, 2041). This high impedance divider assures that the controller 80 draws minimal command signal current.

The controller unit 80 is bistable in operation so that the associated heater remains energized after the command voltage rises above the predetermined level at which the heater is energized. For example where the controller is actuated at a command signal level of about 8 volts, it may be deactuated at 10 volts.

The heater controllers 81-83 are constructed to respond to different low level command voltage signals 50 that staging of the heaters is provided. These controllers are also bistable in operation.

The controllers 80-83 and 92 are failsafe like the controllers 85-87 and 90 in that none of these controllers can be activated when the zone sensing units are disconnected from the logic circuitry. The inputs to the controller units 80-83 and 92 are connected to the positive power supply terminal 110 through the resistor 188 which maintains the voltage level at the conductor 166 at or near the set point voltage level should the zone sensing units be disconnected from the logic circuitry. Hence, none of the controllers 80-83 and 92 can be activated. When the zone sensing units are connected to the logic circuitry, the resistor 188 is shunted by the amplifier output stages 76:: to enable the command voltage at the inputs of the controllers 80-83 and 90 to increase above the set point level.

The secondary heater and cooler controllers 90, 92 can be of the on-off type like the controllers 80, 85, re-

spectively. For example, if the secondary heater 90 is a resistance heater the secondary heater controller 90 may be substantially identical to the heater controller except, of course, that the command voltage level to which the controller 90 responds is of greater magnitude than the level responded to by the'controller 80. The secondary cooler controller 92 is substantially the same as the refrigeration controller except that it is operated at a lower voltage level than the controller 85.

In installations where modulating the secondary heaters or coolers is preferable, the controllers 90, 92 can be of a modulating or proportional type. Circuitry for varying the position of a valve to modulate flow of heating or cooling fluid in response to a command signal is illustrated and described in the above-referenced application to Attridge.

The fact that the controller units are voltage responsive is important because the individual controllers draw minimal amounts of command signal current. This enables the use of one or several stages of heating and/or cooling controllers without requiring circuitry to compensate for changes in the command signal which would be occasioned if the controllers drew any appreciable amounts of command signal current.

THE DAMPER UNITS The preferred system is a constant volume system, i.e., a system in which the volume flow rate of air to each zone remains substantially constant at all times during the operation of the system. In order to maintain the flow rates to each zone constant, zone dampers in the hot and cold air ducts extending to each zone are connected to a common damper actuator so that movement of the zone dampers is correlated. For example, referring to FIG. 1 the zone dampers 48a, 48b controlling the flow of air to zone 1 are each operated by a damper actuator 480. The damper actuator 480 is controlled from the command signal from the zone 1 sensing unit 60 so that the dampers 48a, 48b are positioned according to the command signal levels produced by the Zone 1 sensing unit.

As shown in FIG. 2 the command signal from the zone sensing unit 60 is transmitted to an actuator controlling circuit 96, shown schematically, which operates the actuator 48c in proportion to the command signal. The actuator controlling circuit is connected across the A.C. power supply 103 and has its input connected to the output of the zone sensing unit 60. An output of the circuit 96 is connected to the actuator 48c. The circuit 96 produces an output for operating the actuator 48c which is proportional to the command signal voltage level produced by the zone sensing unit 60. The circuit 96 has a high input impedance so that little command signal current is drawn by it. The actua tor controlling circuit may be of any suitable or conventional construction and is therefore not shown in detail. This circuitry can be constructed, for example, similarly to the refrigeration modulating control circuit disclosed in the cross referenced application of Attridge.

The actuator 48c is preferably hydraulically operated in response to the output from the circuit 96. In the preferred construction, the actuator 48c comprises a constantly operating hydraulic pump which supplies hydraulic fluid to a piston-cylinder type operator. Hydraulic fluid is variably bypassed around the cylinder by a thermally operated valve which governs the fluid pressure in the cylinder according to its position. The

The piston of the operator is drivingly connected to the dampers 48a, 48b and is biased toward a position wherein the hot air damper is opened and the cool air damper is closed. The fluid pressure in the cylinder overcomes the-bias on the piston to move the dampers toward the positions in which the cool air damper is opened and the hot air damper is closed.

The actuator is illustrated and described in more detail in the cross referenced application of Brooks et al. to which reference may be made for a more detailed description.

When the zone sensing unit 60 produces a command signal indicating that zone 1 is at the set point temperature level 12 volts), the circuit 96 conditions the actuator 480 so that the dampers 48a, 48b are each open halfway. Thus half of the air flowing to zone 1 passes through the air cooling heat exchanger and the other half of the air flowing to the zone passes through the air heating heat exchanger. The air heating and air cooling equipment may or may not be functioning, depending on conditions sensed in the other zones, when the sensed air in zone I is at the set point level.

As the zone 1 air temperature decreases from the set point level the command signal is reduced resulting in the cold air damper 48a being moved toward its closed position and the hot air damper 48b moving toward its fully open position. The movement of the dampers 48a, 48b is correlated so that the volume air flow rate to zone 1 remains substantially constant. For example, when the cooling damper 48a is at its one quarter open position the heater damper is at a three quarters open position. When the command signal from the zone 1 sensing unit is at a level indicating that the air temperature in zone 1 is about 85 percent of the set point level, e.g., when the command signal level is at about 10.5 volts, the cooling damper 48a is fully closed and the heating damper 48b is fully open (FIG. 4).

When the zone 1 temperature increases above the set point level, indicating an increased cooling load on zone 1, the cooling damper 48a moves toward its fully open position while the heating damper 48b moves correspondingly toward its closed position. When the zone 1 air temperature is about 15 percent above the set point temperature, the zone 1 command signal is at about 13.5 volts and the cooling damper 48a is fully open with the heating damper 48b fully closed (FIG. 4).

It should be noted that the range of command signal voltages through which the dampers 48a, 48b is modulated can be shifted relative to the set point voltage level by, for example, changing the input impedance of the circuit 96. This may be desirable where the zone associated with the dampers 48a, 48b has an inherently high internal heating load. In such a case the range of command signals to which the actuator 48c responds is shifted so that the cool air damper is fully open at or even below the set point voltage level.

The damper units 49 and 50 are associated with the zone 2 and zone 3 sensing units, respectively, and are constructed and operate the same as the damper unit 48. The unit 49 comprises cold and hot air dampers 49a, 49b, a damper actuator 49c and an actuator controlling circuit 98. The unit 50 comprises cold and hot air dampers 50a, 50b, a damper actuator 50c and an actuator control circuit 100. Since the damper units are connected to their respective zone unit outputs 146, operation of the damper unit for each zone is independent of the operation of damper units of the other zones and is not effected by operation of the zone sensing units of the other zones.

The volume flow rate of air passing through the blower is maintained constant by coordinated movement of the ventilation and return air dampers primarily controlled by the command signal from the warmest zone. The damper actuator 52 is drivingly connected to both the return air damper 34 and the ventilation air damper 32. Operation of the actuator 52 is governed by the damper controller circuit which is connected across the AC. power supply 103 and has its input connected to the output conductor 168 of the logic circuitry (FIG. 2). The actuator 52 and circuit 100 are of the same construction as is described above in reference to the actuator 48c and circuit 96.

The circuit 100 governs operation of the actuator 52 in response to the command signal voltage level from the warmest one of the zones. When the warmest zone temperature is at a predetermined level below the set point temperature level, as will occur when outside air temperatures are low, the actuator 52 positions the dampers 32, 34 so that the ventilation air damper is closed, or in a minimum flow position, and the return air damper is fully open. This enables the air heating units to operate at maximum effectiveness since they are not required to heat the relatively low temperature outside air.

As the temperature of the warmest zone increases toward the set point temperature, the damper controller circuit 100 operates the damper actuator 52 to open the ventilation damper 34 while simultaneously moving the return air damper toward its closed position. These damper movements are proportional to the command signal level from the warmest zone. As noted, the damper movements are correlated so that the volume flow rate of air in the system remains constant.

When the air temperature in the warmest zone increases above the set point temperature level, the dampers 32, 34 are positioned so that the volume of ventilation air flowing into the system is greater than the return air volume. The ventilation air tends to cool the zones. The return air which is not recirculated is exhausted from the system into atmosphere via a suitable exhaust duct, not shown.

When the air temperature in the warmest zone reaches a predetermined level above the set point temperature the ventilation air damper 32 is fully open and the return air damper 34 is closed. This provides a maximum cooling effect from the atmospheric air. In this condition all of the air which has passed through the zones is exhausted to atmosphere from the system.

As is shown in FIG. 4, the range of command voltage levels throughout which the actuator 52 is operated to modulate the proportions of ventilation and return air is disposed relative to the set point voltage level so that the ventilation damper 32 is at its closed or substantially closed position when the command signal from the warmest zone is at about 11 volts, i.e., just below the set point voltage level. This range of operation is desirable because in most cases the zones with all have internal heat sources (e.g., people, machines, etc.) which heat the recirculated air so that the recirculating air thus tends to heat the zones itself. For this reason,

the flow of ventilation air is reduced promptly when all zones are below the set point temperature.

When atmospheric air temperature is too high to satisfy the cooling load on the zones, the air cooling equipment is operated and it is usually undesirable to maintain the ventilation damper 32 fully open since the effectiveness of the air cooling units is reduced where air in the system is not recirculated. Accordingly the damper actuator 52 is operated to close, or substantially close the damper 32 and open the damper 34 so that when the air cooling units operate, the air in the system is composed at least primarily of recirculated air.

In the illustrated embodiment (FIG. 2) the controller unit 85 for the first stage of the cooling equipment operates single throw double pole relay contacts 210 which, when the controller 85 is not activated, connect the input of the damper controller circuit 100 to the logic circuit output conductor 168. When the controller 85 is activated to operate the first stage cooling equipment the contacts 210 are operated to disconnect the input of the damper controller circuit 210 from the conductor 168 and to connect the input of the controller 100 to the low voltage terminal 112 of the power supply. The resultant low voltage level at the input of the controller circuit 100 causes the damper actuator 52 to close the ventilation damper 32 and fully open the return damper 34 so that the air in the system is recirculated.

Whenever the controller 85 is deactivated, the contacts 210 again connect the input of the damper controller circuit 100 to the command signal on the conductor 168 so that the positions of the dampers 32, 34 are again governed by the sensed air temperature in the warmest zone.

Although the contacts 210 are illustrated as being operated by the controller 85, other constructions can be used. For example, the contacts 210 can be replaced by a single throw double pole snap acting switch operated by a thermal power element, not shown, which senses atmospheric air temperature or the temperature of air entering or leaving the blower 22. When the sensed air temperature reaches a predetermined high level, the snap switch is operated to connect the input of the circuit 100 to the low voltage power supply terminal 112 to effect closing of the ventilation air damper 32 and recirculation of the air in the system.

While a single preferred embodiment of the invention has been illustrated and described, the invention is not to be considered limited to the precise construction disclosed. It is intended that all adaptations, modifications and uses of the invention falling within the scope of the appended claims be covered.

What is claimed is:

1. An air conditioning control system for a building defining a plurality of air conditioned zones and having a central air conditioning system comprising a primary air heating means, a secondary air heating means, air cooling means, and air circulating means for directing air from said heating means or cooling means or both to zones in a building, said system comprising:

a. a plurality of signal responsive controller means for controlling operation of said primary heating means, said secondary heating means on said cooling means, respectively;

b. a plurality of zone temperature sensing means each associated with a respective zone and each effective to produce a DC. analog temperature responsive signal in response to sensed zone air temperature;

c. logic circuitry connected between said Zone air temperature sensing means and said controller means, said logic circuitry comprising a first output connected to said controller means for said cooling means and said controller means for said secondary heating means, and a second output connected to said controller means for primary heating means;

d. said logic circuitry comprising first circuit elements for transmitting the maximum zone temperature signal to one of said first and second outputs, and second circuit elements for transmitting the minimum zone temperature signal to the other of said first and second outputs;

e. said controller means operated in response to predetermined voltage levels established at said first and second outputs.

2. The system claimed in claim 1 further including a secondary air cooling means and controller means for controlling operation of said secondary air cooling means, said secondary air cooling controller means connected to said second output of said logic circuitry with said controller means for said primary heating means.

3. The system claimed in claim 1 wherein said signals produced by each of said zone air temperature sensing means range between first and second DC. voltage levels with an intermediate voltage level corresponding to a zone set point temperature, said controller means for said primary heating means operatively energized to operate said primary heating means when air tempera ture sensed in the coolest zone produces a signal voltage between said first D.C. voltage level and said intermediate voltage and which varies from said intermediate voltage by a first predetermined voltage value, said controller means for said secondary heating means operatively energized to operate said secondary heating means when air temperature sensed in the warmest zone produces a signal voltage between said first DC. voltage level and said intermediate voltage and which varies from said intermediate voltage by a second predetermined voltage value.

4. A method of controlling air temperatures in a multiple zone air conditioning system comprising:

a. providing an air conditioning station comprising a primary air heating means, a secondary air heating means and an air cooling means;

b. circulating air from said station to said zones comprising circulating air from said primary and secondary air heating means along individual flow paths to said zones and circulating air from said air cooling means to said zones along individual flow paths;

c. sensing air temperatures in said zones;

d. producing individual D.C. analog command signals indicative of sensed air temperatures in the zones respectively;

e. selecting the command signal from the zone having the lowest sensed temperature;

f. selecting the command signal from the zone having the highest sensed temperature;

g. controlling the primary heating means in response to the command signal from the coolest zone so that said heating means is rendered effective when the temperature sensed in the coolest zone is at a first predetermined level below a set point temperature; and,

h. controlling the secondary air heating means and said cooling means in response to the command signal from the warmest zone;

. controlling said secondary heating means comprising rendering said secondary heating means effective to heat air circulating to said zones when the temperature sensed in the warmest zone is at a second predetermined level below the set point temperature and controlling said air cooling means comprising rendering said cooling means effective to cool air circulating to said zones when the temperature sensed in the warmest zone is a predetermined level above the set point temperature.

5. The method claimed in claim 4 comprising heating said zones by operating said secondary heating means when the temperature of the coolest zone is above the first predetermined level and the temperature of the warmest zone is below said second predetermined level.

6. An air conditioning system comprising:

a. a plurality of air conditioned zones;

b. air heating means;

c. air cooling means;

d. first duct means for separately communicating said heating means and said cooling means to respective zones, said duct means comprising a first plurality of ducts each communicating said air heating means with a respective one of said zones, and a second plurality of ducts each communicating said air cooling means to a respective one of said zones;

e. second duct means comprising at least a return duct for returning air from said zones to said heating and cooling means, and a ventilation air duct for communicating said heating and cooling means to atmospheric air; I

f. temperature sensing means associated with each zone, each sensing means defining an output means producing a DC. analog voltage signal of a given polarity and having a magnitude dependent upon sensed temperature;

g. logic circuitry comprising inputs each of which is electrically connected to a respective one of said temperature sensing means for receiving each of said voltage signals, and first and second outputs for the minimum and maximum signals, respectively, produced by the temperature sensing means;

h. an air heating controller means electrically connected to one of said logic circuit outputs and effective to control operation of said air heating means in response to the signal from said output;

. an air cooling controller means electrically connected to said other logic circuit output and effective to control operation of said air cooling means in response to the signal from said other logic circuit output;

j. a plurality of zone damper means each associated with a respective air conditioned zone and operable to govern the flow of air to said zone through the ducts communicating said zone to said air heating and air cooling means;

k. said zone damper means each comprising:

1. a first damper disposed in a respective one of said first plurality of ducts and movable to control the flow of air in said duct;

2. a second damper disposed in a respective one of said second plurality of ducts and movable to control the flow of air in said duct; and,

3. damper actuator means connected to said first and second air dampers for controlling the proportions of air flowing to said zone from said respective ones of said first and second pluralities of ducts;

4. said damper actuator means electrically connected to the output of said temperature sensing means of said respective zone so that said damper actuator means is operated in response to sensed zone air temperature irrespective of air temperatures in other of said zones; and,

. ventilation damper means for governing the proportions of atmospheric and zone return air flowing to said heating and cooling means via said return and ventilation air ducts, said second damper means comprising a damper actuator electrically connected to said other logic circuit output and operated in relation to the level of the signal produced by said logic circuit output to control the proportions of return and atmospheric air in said system.

7. An air conditioning system as claimed in claim 6 wherein said ventilation damper means is operated to increase the proportion of ventilation air admitted to the system in response to changes in the output signal from the logic circuitry indicating increasing temperature in the warmest zone, and further comprising ventilation damper control means effective to operate said ventilation damper means to a condition wherein minimum ventilation air is supplied to the system in response to the sensed temperature in the warmest zone reaching a predetermined level.

Column 18, line 23 (Claim 6) "second" should'be---venti1ation--.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION v Patent No. 3,788, 386 I Y Dated Jan uary 29, 1974 Inventor(s) j I K Merlin E. Demaray It is ceftified that error ppears in the above-identified patent and that said Letters Patent are hereby corrected as shown below: v

Signed and sealed this 9th day of July 197A.

(SEAL) Attes t; MCCOY M. GIBSON, JR. c. MARSHALL :DANN

Attesting. Officer Commissioner .of Patents- DRM PO-IPSO (1 9 uscomM-oc scan-ps9 I I Q U.S. GOVEINMENI IRINHNG OFFICE l8! 0-38-314 

1. An air conditioning control system for a building defining a plurality of air conditioned zones and having a central air conditioning system comprising a primary air heating means, a secondary air heating means, air cooling means, and air circulating means for directing air from said heating means or cooling means or both to zones in a building, said system comprising: a. a plurality of signal responsive controller means for controlling operation of said primary heating means, said secondary heating means on said cooling means, respectively; b. a plurality of zone temperature sensing means each associated with a respective zone and each effective to produce a D.C. analog temperature responsive signal in response to sensed zone air temperature; c. logic circuitry connected between said zone air temperature sensing means and said controller means, said logic circuitry comprising a first output connected to said controller means for said cooling means and said controller means for said secondary heating means, and a second output connected to said controller means for primary heating means; d. said logic circuitry comprising first circuit elements for transmitting the maximum zone temperature signal to one of said first and second outputs, and second circuit elements for transmitting the minimum zone temperature signal to the other of said first and second outputs; e. said controller means operated in response to predetermined voltage levels established at said first and second outputs.
 2. The system claimed in claim 1 further including a secondary air cooling means and controller means for controlling operation of said secondary air cooling means, said secondary air cooling controller means connected to said second output of said logic circuitry with said controller means for said primary heating means.
 2. a second damper disposed in a respective one of said second plurality of ducts and movable to control the flow of air in said duct; and,
 3. damper actuator means connected to said first and second air dampers for controlling the proportions of air flowing to said zone from said respective ones of said first and second pluralities of ducts;
 3. The system claimed in claim 1 wherein said signals produced by each of said zone air temperature sensing means range between first and second D.C. voltage levels with an intermediate voltage level corresponding to a zone set point temperature, said controller means for said primary heating means operatively energized to operate said primary heating means when air temperature sensed in the coolest zone produces a signal voltage between said first D.C. voltage level and said intermediate voltage and which varies from said intermediate voltage by a first predetermined voltage value, said controller means for said secondary heating means operatively energized to operate said secondary heating means when air temperature sensed in the warmest zone produces a signal voltage between said first D.C. voltage level and said intermediate voltage and which varies from said intermediate voltage by a second predetermined voltage value.
 4. said damper actuator means electrically connected to the output of said temperature sensing means of said respective zone so that said damper actuator means is operated in response to sensed zone air temperature irrespective of air temperatures in other of said zones; and, l. ventilation damper means for governing the proportions of atmospheric and zone return air flowing to said heating and cooling means via said return and ventilation air ducts, said second damper means comprising a damper actuator electrically connected to said other logic circuit output and operated in relation to the level of the signal produced by said logic circuit output to control the proportions of return and atmospheric air in said system.
 4. A method of controlling air temperatures in a multiple zone air conditioning system comprising: a. providing an air conditioning station comprising a primary air heating means, a secondary air heating means and an air cooling means; b. circulating air from said station to said zones comprising circulating air from said primary and secondary air heating means along individual flow paths to said zones and circulating air from said air cooling means to said zones along individual flow paths; c. sensing air temperatures in said zones; d. producing individual D.C. analog command signals indicative of sensed air temperatures in the zones respectively; e. selecting the command signal from the zone having the lowest sensed temperature; f. selecting the command signal from the zone having the highest sensed temperature; g. controlling the primary heating means in response to the command signal from tHe coolest zone so that said heating means is rendered effective when the temperature sensed in the coolest zone is at a first predetermined level below a set point temperature; and, h. controlling the secondary air heating means and said cooling means in response to the command signal from the warmest zone; i. controlling said secondary heating means comprising rendering said secondary heating means effective to heat air circulating to said zones when the temperature sensed in the warmest zone is at a second predetermined level below the set point temperature and controlling said air cooling means comprising rendering said cooling means effective to cool air circulating to said zones when the temperature sensed in the warmest zone is a predetermined level above the set point temperature.
 5. The method claimed in claim 4 comprising heating said zones by operating said secondary heating means when the temperature of the coolest zone is above the first predetermined level and the temperature of the warmest zone is below said second predetermined level.
 6. An air conditioning system comprising: a. a plurality of air conditioned zones; b. air heating means; c. air cooling means; d. first duct means for separately communicating said heating means and said cooling means to respective zones, said duct means comprising a first plurality of ducts each communicating said air heating means with a respective one of said zones, and a second plurality of ducts each communicating said air cooling means to a respective one of said zones; e. second duct means comprising at least a return duct for returning air from said zones to said heating and cooling means, and a ventilation air duct for communicating said heating and cooling means to atmospheric air; f. temperature sensing means associated with each zone, each sensing means defining an output means producing a D.C. analog voltage signal of a given polarity and having a magnitude dependent upon sensed temperature; g. logic circuitry comprising inputs each of which is electrically connected to a respective one of said temperature sensing means for receiving each of said voltage signals, and first and second outputs for the minimum and maximum signals, respectively, produced by the temperature sensing means; h. an air heating controller means electrically connected to one of said logic circuit outputs and effective to control operation of said air heating means in response to the signal from said output; i. an air cooling controller means electrically connected to said other logic circuit output and effective to control operation of said air cooling means in response to the signal from said other logic circuit output; j. a plurality of zone damper means each associated with a respective air conditioned zone and operable to govern the flow of air to said zone through the ducts communicating said zone to said air heating and air cooling means; k. said zone damper means each comprising:
 7. An air conditioning system as claimed in claim 6 wherein said ventilation damper means is operated to increase the proportion of ventilation air admitted to the system in response to changes in the output signal from the logic circuitry indicating increasing temperature in the warmest zone, and further comprising ventilation damper control means effective to operate said ventilation damper means to a condition wherein minimum ventilation air is supplied to the system in response to the sensed temperature in the warmest zone reaching a predetermined level. 